In existent offshore wind turbine installations different foundations are common: Monopile foundations consist of a steel pile which is driven into the seabed. Gravity foundations consist of a large base constructed from either concrete or steel or a combination, which rests on the seabed. The wind turbine is dependent on gravity to remain erect. Tripod foundations consist of three legs comprising piles on each end which are driven into the seabed. This type is generally used at deeper depths.
Furthermore jacket structures have been used for offshore wind turbine installations. Traditionally the jacket structure is a steel structure where all parts are welded as three dimensional connections at central connection joints. It is advantageous that the jacket structure is a relative low-cost structure, it is easy to construct and has a high stiffness-to-material use ratio. Furthermore it is efficient in positions with high wave loads.
In EP 2 067 914 A2 a jacket structure is disclosed, where the jacket is built of a number of types interconnected by means of casted independent central nodes. The pipes and casted nodes are welded together. However, it is difficult to transport large jacket structures to a proposed construction site, therefore jacket structures are prefabricated as modules, which are welded together at the installation place. On the other hand it is difficult to weld a large number of connections at the construction site with the necessary quality.